The present invention relates to an image forming method by which images are recorded with a laser scanning optical system.
An image forming method is well known which is characterized in that: the surface of a photoreceptor drum is scanned by a single or a plurality of laser beams, and information according to a recording signal is recorded on the photoreceptor. The following scanning means is applied to the aforementioned image forming method: a laser beam is reflected by a rotating polygonal mirror which is rotated by a motor at constant speed; the reflected beam is transmitted through an f.theta. lens; and a minute spot is formed on the surface of the photoreceptor so as to be scanned in the form of dot exposure.
The following image forming means is applied to the aforementioned image forming method: the photoreceptor drum is rotated by a motor at constant speed; the shaft of the photoreceptor drum is disposed in parallel with the scanning direction of the laser beam, so that auxiliary scanning is conducted by the rotation of the photoreceptor drum; and due to the aforementioned primary and auxiliary scanning, a latent image is formed on the circumferential surface of the photoreceptor drum which has been uniformly charged.
In the aforementioned image forming method, image quality is determined by a dot diameter of a toner image which is formed when a latent image made by the laser beam is developed.
In order to reproduce a half-tone image the following method is applied:
A pulse width modulation signal is obtained by a comparison circuit from a reference signal of a triangular wave, the period of which is the same as the recording signal of an analog video signal; a semiconductor laser is driven by the aforementioned pulse width signal; a laser beam emitted from the semiconductor is projected in the photoreceptor so that a latent image is formed; and the length of the latent image in the scanning direction is varied according to the aforementioned pulse width, so that a minute rectangular latent image of multi-value can be obtained.
In another image forming method, the distribution of light intensity of a laser beam spot on a photoreceptor drum is approximately the same as a Gaussian curve. Accordingly, when a portion not less than a predetermined threshold value (s) has been developed, electrical potential of toner adhesion us changed. Consequently, when the light intensity of a spot is varied during the process of latent image formation, the diameter of a dot of a toner image (referred to as a toner dot, hereinafter) is also varied. As shown in FIG. 14-A, when the light emitting time is extended, the diameter is increased not only in the primary scanning direction but also in the auxiliary scanning direction.
In this case, a half-tone image is reproduced in the following manner: a modulation signal is obtained which varies in accordance with the density data corresponding to a pixel; a semiconductor laser is driven by this intensity modulation signal; a laser beam emitted from the semiconductor laser, the intensity of the laser beam is varied, is projected on the surface of the photoreceptor drum so that a latent image is formed; and then, the diameter of the latent image dot is varied according to the aforementioned light intensity, so than a minute circular or oval latent image of multi-value can be obtained.
However, in the case of the aforementioned image forming method, the configuration of a spot of a conventional laser beam projected onto the surface of a photoreceptor, is an oval, the .major axis of which is in the auxiliary scanning direction. Therefore, as shown in FIG. 14-B, toner dots are connected in the auxiliary direction, so that longitudinal stripes appear on the image. Accordingly, in the case of a half-tone image, the quality of an image is deteriorated, especially in the case of an image of human's skin. Especially, in the case of pulse width modulation of 2 pixel unit, it is disadvantageous in that stripes are remarkable. In the region of a character image or line image (This region will be referred to as a character region, hereinafter.), unlike a half-tone image region (This region will be referred to as a half-tone region, hereinafter.), the frequency at which a laser beam spot passes through the same place, is low so that the change of electrical potential is small. For that reason, when image formation is conducted under the condition that the writing light amount of the semiconductor laser is the same, the diameter of the toner dot in the character region and that of the toner dot in the half-tone region are different. Accordingly, when the writing light amount to maintain the gradation of the half-tone region appropriately, is adopted, the gradation property of the character region is lowered, and sufficient density can not be provided. Further, when the writing light amount of the semiconductor laser is increased in order to obtain sufficient density of the character region, saturation is early caused in the half-tone region, and the gradation property is deteriorated.
The first object of the present invention is to provide an image forming method by which an image is formed when scanning is conducted by a laser beam which has been pulse-width-modulated or intensity-modulated, wherein the image forming method is characterized in that stripes caused when toner dots are connected with each other, do not occur in the auxiliary scanning direction; the gradation property is improved in both the character and half-tone regions; and a wide dynamic range is provided so that an image of high quality can be obtained.
Further, in the present invention, distribution of adjoining pixels is reflected in density distribution of objective recording pixels so that high quality recording can be conducted. The present invention relates to a color image forming apparatus in which: one matrix pixel image data is divided into small pixels m.times.n (the width.times.the length) considering the adjoining pixel data, and after that, the center of gravity of each line is found; the phase of the reference wave is deviated according to the center of gravity; and dot recording composed of n small scanning lines is conducted by the modulated signal of the pixel density data modulated by the reference wave signal so that a character and a halftone image can be reproduced. The recording apparatus of the present invention is used for a printing apparatus or a displaying apparatus.
In the field of an image forming apparatus based on an electrophotography, an image on an original is read as image signals by a scanner, and the image signals are subjected to gradation correction, A/D conversion and shading correction to be image density data. The image density data are modulated by reference wave signals so that digital images having therein reproduced half tone images may be obtained.
With regard to image signals obtained by reading an image on a original by a scanner, an edge portion of the image is read as a half tone image density due to an aperture of a solid image sensor incorporated in the scanner. When image density data obtained from the image signals cause a latent image on a photoreceptor, recording pixels corresponding to edges of the latent image are recorded on an average within recording pixels when density is medium. Therefore, sharpness of the image is deteriorated for recording. For this problem, there have been known MTF correction methods wherein a differential filter or a Laplacian filter is used for image signals for sharpening images. In these methods, however, only edge portions of an image are sharpened and uniformity of halftone images is relatively deteriorated.
Even when interpolation characters or graphics are formed by CG or font data, on the other hand, the same problem still happens. Namely, when interpolation data interpolate edge portions smoothly by means of medium density, recording pixels corresponding to the edge portions are recorded in pixels as the average density. Therefore, the sharpness of recorded images is deteriorated. For the above reason, medium density processing that works effectively on the edge portions of an image is required.
As a method to form a latent image in the manner described above, the following technical means is well known: a minute spot is formed by a single or a plurality of laser beams modulated by a recording signal; and dot exposure is conducted when a photoreceptor surface is scanned by the aforementioned minute spot.
For that reason, it is necessary to perform the medium density processing which substantially acts on the edge portion of an image. However, when the medium density processing is carried out at each color in the case of color image formation, the following problems are caused: image tone is varied; and characters are blurred.
The second object of the present invention is to provide an image forming method by which images are formed when scanning is performed by a modulated laser beam, wherein the image forming method is characterized in that: the occurrence of undesired strips can be prevented; and the sharpness is improved, so that images of high quality can be obtained.